Soil & Tillage Research 223 (2022) 105490

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Soil & Tillage Research

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Review

Physical, chemical and biological subsoiling for sustainable agriculture

Tangyuan Ning a, b, *, Zhen Liu a, Hengyu Hu a, Geng Li a, Yakov Kuzyakov b, c
a
State Key Laboratory of Crop Biology, College of Agronomy, Shandong Agricultural University, Taian 271018, China
b
Department of Agricultural Soil Science, Department of Soil Science of Temperate Ecosystems, University of Gottingen, Gottingen 37077, Germany
c
Peoples Friendship University of Russia (RUDN University), 117198 Moscow, Russia

A R T I C L E I N F O A B S T R A C T

Keywords: Subsoil degradation – mainly through strong compaction (to density > 1.6 g cm− 3) by intensive agriculture with
Nutrient acquisition and recycling heavy machinery – is a global problem for soil health, crop production, carbon sequestration, and the envi­
Tillage depth and periodicity ronment. Subsoiling is a field measure to improve the physical, chemical, and biological properties of the soil
Deep-rooted crops
below the common plowing depth to increase crop yields, water and nutrient use efficiency, economic benefits,
Fungal inoculation
and ecological functions. Traditionally, physical (phy-), chemical (chem-) or biological (bio-) subsoiling ap­
Earthworms and biopores
Land use practices proaches are used to recover degraded subsoils, whereas their combination was disregarded. This review sum­
marizes current knowledge on subsoiling approaches and their effects on soil properties, crop production, carbon
storage and other ecosystem functions. A meta-analysis showed that phy-subsoiling boosts crop yields by 19 % on
average, with a temporal decrease in organic carbon content in the topsoil compared to no-till cultivation. Phy-
subsoiling is necessary but not sufficient to completely resolve tillage pan compaction problems. Bio- and chem-
subsoiling combined with phy-subsoiling very efficiently increase the full range of soil fertility properties for a
long duration, raising crop yields and strengthening economic benefits because the combination retains the
advantages while reducing the shortcomings of individual subsoiling approaches. Thus, farmers should upgrade
phy-subsoiling with bio-approaches, including the use of deep-rooted crops and straw incorporation, and chem-
subsoiling modes, including manuring and liming.

1. Introduction deeper than 100 cm (Fan et al., 2016). Despite the urgent need to find
unexplored water and nutrient sources for sustainable agriculture, one
Subsoil (soil below the common tillage depth, > 30 cm) degradation, of the largest nutrient reservoirs – the subsoil – has been widely
especially strong compaction, is a worldwide problem in all agricultural neglected (Kautz et al., 2013).
fields (Fig. 1) (Nachtergaele et al., 2010; Spoor et al., 2003) and even in One or more compacted soil layers can be detected in the upper
some forest plantations (Horn et al., 2004). Subsoil compaction creates subsoil in most agricultural fields, which is mainly the result of intensive
unfavorable physical, chemical and biological conditions, restrains root farming practices, including heavy machinery and short crop rotations
penetration and distribution and water and nutrient storage, and (Guaman et al., 2016; Raper, 2005). Strong soil compaction is one of the
consequently reduces crop yields (Feng et al., 2018; Guaman et al., main forms of field degradation in Central and Eastern Europe and has
2016; Hartmann et al., 2008; Wang et al., 2020b). Subsoils between the heavily affected over 11 % of the total land area there (Görlach et al.,
30–100 cm depth can store approximately 50 % of the total nitrogen 2004). Bulk densities up to 1.8 g cm− 3 were recorded at depths between
stocks, 25–70 % of the total phosphorus stocks, and more than 50 % of 20 and 30 cm (Gameda et al., 1994). Such soil compaction limits root
the water in cropland (Kautz et al., 2013; Ma and Song, 2016). More proliferation and distribution, thus restricting the availability of water
than two-thirds of the soil organic carbon (SOC) in the entire land area of and nutrients to crops to that only available in the topsoil (Feng et al.,
the world is stored within the 30–200 cm soil layer (Batjes, 1996; 2018). Corn yields in Eastern Canada were reduced by 18–27 % by soil
Jobbágy and Jackson, 2000). Nearly 75 % of inorganic carbon (C), compaction under optimal weather conditions and by 55–86 % under
mainly CaCO3, is located at 30–100 cm (Batjes, 1996; Raza et al., 2020; adverse weather conditions (Gameda et al., 1994). Soil compaction
Zamanian et al., 2018). reduced crop yield by 6–12 % in Europe, and the losses in product value
Most crops have root systems which are potentially able to grow were €487 million in Germany and €713 million in France (Schjønning,

* Corresponding author at: State Key Laboratory of Crop Biology, College of Agronomy, Shandong Agricultural University, Taian 271018, China.
E-mail addresses: ningty@163.com, tning@uni-goettingen.de (T. Ning).

https://doi.org/10.1016/j.still.2022.105490
Received 30 July 2021; Received in revised form 2 May 2022; Accepted 13 July 2022
Available online 21 July 2022
0167-1987/© 2022 Elsevier B.V. All rights reserved.
T. Ning et al. Soil & Tillage Research 223 (2022) 105490

2018). This calls for efficient and low-cost subsoil rehabilitation tech­ There are many differences in the physical, chemical, and biological
niques to guarantee food safety and mitigate climate change effects, e.g., properties between the subsoil and topsoil (Table 1). Bulk density,
droughts (Hartmann et al., 2008; Lal, 2004). compaction, mechanical resistance, and aggregate stability are in most
According to 1146 publications, the average soil depth studied in the cases higher in the subsoil than in the topsoil (see Table 1 for references).
past 30 years was 27 cm, but it was only 24 cm from 2004 to 2019 (Yost Correspondingly, pore size, air permeability, and WHC per volume in
and Hartemink, 2020). Consequently, the subsoil problems mentioned the subsoil are mostly lower than those in the topsoil. The global SOC
above are disregarded in most studies. It is much more complex and storage in the subsoil from 30 to 200 cm is 2.4 times greater than that in
expensive to alleviate subsoil versus topsoil compaction (Guaman et al., topsoil (Batjes, 1996), whereas the storage of inorganic C in subsoil is
2016). To comprehensively understand the technological advances of 4–7 times greater than that of topsoil (Lal, 2004; Zhang et al., 2015,
subsoil improvement, we define subsoiling as all field management 2018). The 14C age of SOC in subsoil is much older (Angst et al., 2016).
practices that affect the physical, chemical, and biological properties of The root biomass, microbial diversity, enzyme activities, earthworm
the subsoil (depth > 30 cm, mostly the 30–100 cm depth) to increase numbers, and phospholipid fatty acids are also mostly lower in the
rooting depth, resource use efficiency, crop yield, economic benefits, subsoil.
and ecological functions. Water, carbon, and nutrients (mostly N, P, K, Ca, and Mg) in subsoil
Three groups of approaches are available today, namely, physical are essential for high and stable yields (Table 2). Subsoil can contribute
(phy-), biological (bio-), and chemical (chem-) subsoiling (Guaman more than two-thirds of the N, P, and K nutrients, especially when the
et al., 2016; Lynch and Wojciechowski, 2015; Wang et al., 2019). topsoil is nutrient depleted (Kautz et al., 2013). Deep rooting provides
“Subsoiling” in previous papers refers mostly to the process of soil tillage numerous benefits, especially during drought or in low-fertility soils
performed by a tool inserted into the soil to a depth of at least 35 cm (Gao et al., 2016).
(Raper, 2005; Raper et al., 2005). This is a typical kind of phy-subsoiling
according to our definition.
2.2. Necessity for roots to explore subsoil
This review summarizes the current knowledge regarding these three
modes of subsoiling: phy-, chem-, and bio-subsoiling. After a short
Most crops have a maximal root depth between 100 and 200 cm (Fan
overview of the key differences between subsoil and topsoil, we present
et al., 2016; Min et al., 2014; Yamaguchi and Tanaka, 1990). The cu­
1) the detailed methods of phy-, chem-, and bio-subsoiling and their
mulative root distribution of most crops is logarithmic with respect to
main advantages and shortcomings; 2) subsoil applications for best
soil depth (Fig. 2). Forty-five percent of roots of alfalfa, potato, and
agronomic practices; and finally, 3) discuss future applications and
chickpea, 40 % of those of wheat, soybean, pea, and canola, 25 % of
study areas related to subsoiling.
those of barley, maize, oat, and cotton, and approximately 10 % of those
of rice grow in the subsoil – below 30 cm. The soil depth down to which
2. Subsoil properties
80 % of the roots are localized is 80 cm for alfalfa, 40–60 cm for most
other crops, and only 25 cm for rice (Fig. 2).
2.1. Specifics of subsoil properties compared to those of topsoil
Two or even 3 tillage pans are often present in topsoil and subsoil,
which can be caused by long-term tillage and high axle loads (Guaman
In arable farming systems, the term “topsoil” refers to the epipedon,
et al., 2016; Martinez et al., 2019; Raper, 2005; Schjønning et al., 2016).
the surface or Ap horizon, which is generally 15–30 cm in depth. This
Two pans were found in a maize field: one was a disk pan and the other
layer is highly influenced by fertilization, crop roots, and associated
was a plow pan (Fig. 3 right). The upper pan was located mostly in the
microbial activities and is mixed and frequently intensively disturbed by
5–15 cm soil depth, and the deep pan was in the 20–30 cm soil depth
tillage. Topsoil is therefore enriched in nutrients and generally charac­
(Fig. 3 left). The penetration resistance in the tillage pan can measure up
terized by active nutrient transformations between organic and mineral
to 3.5 MPa, but most crop roots cannot grow into soil with values greater
forms compared to the subsoil. The term “subsoil” refers to the strata
than 2 MPa (Martinol and Shaykewich, 1994). These pans limit the root
below the topsoil, including the E, B, and, in some cases, C horizons.
system to the upper 10–15 cm and thus decrease the amount of available

Fig. 1. World map of subsoil compaction. The most common causes for subsoil compaction are the use of heavy machinery and inappropriate tilling practices, mostly
in industrialized agriculture, and a strong concentration of livestock with overgrazing, particularly in drier climates around water-points. The map is from Nach­
tergaele et al. (2010).

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Table 1
Differences in subsoila and topsoil (Ap) properties.
Soil properties Subsoil depth Changes + Units References
(cm)

Physical
Bulk density 20–40 ↑3–26 % Jug et al. (2019);Lu and Liao (2017);Qin et al. (2008);Xu and
Mermoud (2001)
Plow pan 20–60 Less Dong et al. (2017);Raper (2005)
Large pores 20–40 ↓20 % Carmeis Filho et al. (2016)
Aggregate size 20–40 ↑12–20 % Zhang et al. (2017);Zhang et al. (2014)
Clay content 30–60 ↓3 % in Stagnosol -↑26 % in Jug et al. (2019)
Gleysol
Soil compaction 20–80 ↑2.18–2.86 times Cai et al. (2014);Jug et al. (2019)
Porosity 20–40 ↓7–12 % He et al. (2019);Jug et al. (2019)
Soil resistance 20–40 ↑3 % Akinci et al. (2004)
Air permeability 20–40 ↓31–47 % Colombi et al. (2019)
WHC 20–40 ↓20 % -↑13 % Colombi et al. (2019)
Soil water contributions to crop 20–200 ↑15 % (Ma and Song, 2016)
Chemical
SOC content 20–40 ↓40–68 % Zhang et al. (2017)
Colombi et al. (2019)
SOC storage 30–200 ↑1.4 times Batjes (1996)
SIC storage 30–200 ↑4–7 times Lal (2004);Zhang et al. (2015);Zhang et al. (2018)
CEC 20–25 ↓5 %-↑12 % Blumenschein et al. (2019)
C/N 20–30 ↓4 % Ussiri and Lal (2009)
EC 30–60 ↑1.2 times (Grevers and de Jong, 1993)
14
C content 35–60 ↓8.5 % Angst et al. (2016)
14
C age 35–60 ↑7.5 times Angst et al. (2016)
Total and available nutrients 30–200 ↑0.8–2.5 times Ge et al. (2020);Wen et al. (2020)
storage
Biological
Root biomass 35–60 ↓44 % Angst et al. (2018)
Microbial biomass 20–40 ↓17 % Dick et al. (1988);Yan et al. (2019)
Microbial diversity 20–40 ↓ Yan et al. (2019)
MBC 20–30 ↓27–39 % (Balota et al., 2014)
Fungal/bacterial C 20–40 ↑23–45 % Angst et al. (2018)
Enzyme activities 10–20 ↓1–27 % Dick et al. (1988);Ji et al. (2014)
Arbuscular mycorrhizal fungi 30 Claroideoglomeraceae ↑2.53 times Sosa-Hernández et al. (2018)
Diversisporaceae ↓83 %
Phospholipid fatty acids 60 ↓89 % Liang et al. (2019)
a
Topsoil refers to the epipedon, the surface or Ap horizon, generally 15–30 cm in depth. Subsoil refers to deeper soil strata: that is, the E, B, and, in some cases, C
horizons. For the comparison, it was assumed that in contrast to the Ap horizon, the subsoil was undisturbed by any agronomic measures. The values reflect the
decrease (↓) or increase (↑) of the parameter per unit of subsoil volume compared to that of the topsoil, except for SOC storage, SIC storage, and total and available
nutrients storage, which are expressed per total subsoil volume. Because of the limited study of related subsoiling, some values are given as examples. The primary
parameters (affecting many other parameters) are underlined. The most agronomically important properties are in italics.

water and nutrients for crops (Raper, 2005). No-till, which is widely
Table 2
used to minimize tillage pan formation, has no effects on subsoil
Sources of water, carbon and nutrients in topsoil and subsoil.
compaction resulting from heavy axle loads (Håkansson and Reeder,
Sources Topsoil Subsoil References 1994). Whereas no-till is not suitable and cannot alleviate soil
Water Wang et al. (2010);Yang et al. (2015); compaction, subsoiling is vitally needed to break or remove the pans and
Precipitation XXX X Guo et al. (2016a);Wu et al. (2016); ( to create more suitable environmental and resource conditions for crop
Groundwater (X) (XXX) Ma and Song, 2016)
root growth and yield formation (Table 1 and Table 2). This makes
Irrigation (XXX) (X)
Carbon
breaking the pans between the topsoil and subsoil and making these
Aboveground XXX X Sokol et al. (2019);Pisani et al. (2015) layers into a more continuous body very important for crop productivity.
litter
Roots XX XXX Angst et al. (2016)
3. Subsoiling approaches and effects on soil properties
Rhizodeposition X XXX Mwafulirwa et al. (2017)
Organic fertilizers (XXX) (X) Das et al. (2017)
Nutrients There are three modes of subsoiling: phy-, chem-, and bio-subsoiling
Mineral N XXX X Tian et al. (2017) (Table 3). In this section, we discuss the definitions, aims, most common
fertilization
approaches, and advantages and shortcomings of each subsoiling mode.
NO–3 leaching X XXX Wang et al. (2018)
N2 fixation XX X Xu et al. (2019)
Atmospheric XXX Böhme et al. (2003)
deposition 3.1. Physical subsoiling
Mineral PK (XXX) (X) Poeplau et al. (2018);Hobley et al.
fertilization (2018) Physical (Phy-)subsoiling has a long history and is widely used on
Organic fertilizers (XXX) (X) Wang et al. (2016a)
farms today. Phy-subsoiling mechanically disrupts compacted layers or
Rainfall XXX X Xu et al. (2015)
horizons, decreases bulk density, or directly changes other physical
The number X represents the relative importance of the sources for topsoil and properties below the common plow depth to create better conditions for
subsoil. (X) brackets indicate “if present”. crop rooting. Phy-subsoiling is designed to mechanically destroy dense
and compacted layers or horizons, increase the volume for root growth,
and improve the conditions for water penetration and storage, nutrient

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T. Ning et al. Soil & Tillage Research 223 (2022) 105490

Fig. 2. Cumulative root distribution as a function of soil depth (down to 200 cm) for 12 main crops. Number after each crop: the reported maximum rooting depth
(cm); n: number of references. Horizontal lines: the soil depth in which 80 % of all roots are located. Vertical lines: the cumulative root percentage in the topsoil
(0–30 cm). The data of horizontal and vertical lines are calculated according to the equations fitted based on the experimental points (equations are presented on the
Figure). All regression equations are significant at least at p < 0.01. Note the logarithmic scale of the depth axis. Rice: (Yamaguchi and Tanaka, 1990; Laila and
Waluyo, 2016; Drescher et al., 2020). Potato: (Yamaguchi and Tanaka, 1990; Guaman et al. (2016); Krystyna et al., 2017; Reyes-Cabrera et al., 2016; Puertolas et al.,
2014; Ahmadi et al., 2011. Cotton: Min et al. (2014); Ning et al., 2015; Chen et al., 2018; Zhang et al. (2017). Other crops: derived from a review of crop root
distribution by Fan et al. (2016).

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Fig. 3. Hard pans in a maize field (left) and penetration resistance in soil profile (right). The photo on left is modified from Raper (2005) and the graph on right is
taken from 3 studies (listed as references [1], [2], and [3]). The references are: 1 for Guaman et al. (2016), 2 for Schjønning et al. (2016), and 3 for Martinez et al.
(2019). In the photo, two pans are present, a disk pan in 5 cm depth and a plow pan in 20 cm depth. The subsoiling can break these two pans and enable maize roots
to grow deeper into the subsoil. In the graph, black arrow represents disk pan, and red arrow represents plow pan.

Table 3
Overview of physical-, chemical-, and biological-subsoiling modes.
Name Aim Depth Yearly Advantages Disadvantages References
(cm) interval

Phy-subsoiling
Deep plowing Dens, Pan, 30 2–3 Loosens soil, crop benefits Recompaction of the soil, energy Ji et al. (2014)
Root, Water consumption Tian et al. (2014)
Straight shank Dens, Pan, 33–50 2–3 Loosens soil without mixture of soil layers, High energy consumption, machine Raper (2005)
subsoiler Root, Water crop benefits, increases SOC, can be used compaction Tian et al. (2014)
during crop growth Ogbeche et al. (2018)
Bentleg subsoiler Dens, Pan, 33–48 2–3 Loosens soil without mixture of soil layers, Energy consumption, machine Raper et al. (2005)
Root, Water crop benefits, can be used during crop compaction
growth
Paraplow Root, 45 1 Vertical rooting depth, high air Energy consumption, machine Hamilton-Manns et al.
Water, O2 permeability compaction (2002)
Sojka et al. (1997)
Vertical rotary Root, 40 1 Increases water use efficiency, loosens soil Reduces soil structure stability and Zhang et al. (2020)
subsoiling Water, SOC storage over the long term
Dens, O2
Deep ripping Dens, Pan, 70–75 4–5 Decreases soil strength and compressibility, Coincides with a continuous Canarachea et al.
Root, Water even in the Bt horizon redeterioration of newly formed (2000)
aggregates
Chem-subsoiling
Subsoil manuring NPK, Root, 30 4 Increases crop root length and density and Machine dependent Wang et al. (2020a)
C soil macroaggregate formation Peries (2013)
Lime incorporation pH, NPK 30–60 2–5 Reduces subsoil acidity and improves plant Machine dependent Blumenschein et al.
growth (2019)
Bio-subsoiling
Deep-rooted crops or Root, > 50 1 Creates or enlarges existing vertical Effects are slower and weaker than (Lynch and
cover crops Water, NPK biopores, increases crop root density those of phy-subsoiling, limited by Wojciechowski, 2015)
abiotic factors Guaman et al. (2016)
Schjønning et al.
(2015)
Straw burying Root, NPK 30 1 Increases SOC and STN stocks, increases Dependent on nutrient level and Tian et al. (2019)
soil microbial processes anaerobic conditions in subsoil, Yang et al. (2019)
lacking all-in-one machine
Earthworms Root, NPK, > 30 / Increases biopores, enzyme activities and Accelerates SOC turnover Hoang et al. (2017)
Dens microbial activities
Arbuscular NPK, / / Causes formation of aggregates, increases Functioning of AM fungal communities Lino et al. (2018)
mycorrhiza fungal Water, C soil nutrients and SOC in the subsoil is not well-known Sosa-Hernández et al.
inoculation (2019)

C: carbon storage, Dens: decreased density, NPK: uptake of NPK from subsoil, O2: aeration, Pan: against plow pan, pH: control pH, Root: better root growth, Water:
storage of more water in subsoil for crop use. The “yearly interval” means that the next subsoiling event should be used after year “N”. “/” means no data or no
recommendation.

availability, and aeration (Table 3 and Fig. 4). surface, and ii) lateral compaction occurs at the border to the soil, which
Phy-subsoiling is characterized by the depth, area and volume of the is not involved in mechanical movement (Fig. 4). The subsoiling char­
cut or inverted soil (Sun et al., 2018). The bulk density in the disturbed acteristics are affected by the main components of the subsoiler: the
area may decrease from more than 1.6 cm− 3 to 1.2 g cm− 3 because i) the toolframe, shanks, and points (Fig. 4). The subsoiled volume is increased
soil is loosening and a portion of the soil is moved above the previous by the working depth and by a subsoiler with a wider point or higher lift

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Fig. 4. Illustration of soil properties after physical subsoiling. The three main
components of a subsoiler are the toolframe, the shanks, and the points. A
shank comprises a leg and a point. After subsoiling, the pan is broken, and the
soil density in the disturbed area decreased because i) a part of the soil is
ejected on the surface and ii) lateral compaction (1.8 g cm− 3) to the interfaces
of nearby area which increase the space for the disturbed soil. The density
decrease in the subsoiled area is affected by the ratio of the average height of
elevated area to subsoiling depth. The subsoiled volume is affected by shank
structure, for example point width and wing lift height, shank distance, and
subsoiling depth (see Fig. 4).

height (Askari et al., 2017; Ogbeche et al., 2018). Phy-subsoiling is Fig. 5. Working effects of main types of physical subsoilers. Red double-headed
mainly performed using subsoilers with straight or curved shanks with arrows: maximum subsoiling depths, corresponding to the work subsoiler active
wings (Hamilton-Manns et al., 2002; Ogbeche et al., 2018; Raper et al., area. Purple double-headed arrow: wing lift height. The subsoiled area is
2005; Tian et al., 2014; Zhang et al., 2020). A straight shank with wings affected by the shank structure and work depth. Straight shanks with wings
can produce a greater subsoiled volume than that without wings (Fig. 5). enable more subsoiled area than those without wings. For curved shanks,
Deep phy-subsoiling with curved shanks causes some local compaction subsoiling too deeply causes local compaction of the topsoil. Blue and black
of the topsoil (Fig. 5). single arrows: direction of subsoiler movement and soil compaction, respec­
tively. Figure redrawn according to (Weill, 2015).
The depth, working machine, and intervals of phy-subsoiling should
be decided according to specific aims (Canarachea et al., 2000; Raper
et al., 2005). The usual phy-subsoiling depth is 30–50 cm based on the Mechanically loosened soil begins to recompact after 1–3 years
place of the compaction layer and the crop root requirements (Table 3). (Schneider and Don, 2019). Therefore, the positive effects of
The phy-subsoiling depth increases when the pan is deeper. The subsoil phy-subsoiling rapidly decreased, requiring repetition every 2–4 years
depth for deep rooting crops, such as wheat and alfalfa, needs to be (Canarachea et al., 2000; Tian et al., 2016). Most phy-subsoiling modes,
increased to 40–50 cm. such as vertical rotary subsoiling and deep ripping, will accelerate
The phy-subsoiling interval should be 1–2 or even 4–5 years organic matter decomposition over a short time (Table 3) (Canarachea
depending on tillage modes (Table 3). The phy-subsoiling intervals can et al., 2000; Fontaine et al., 2007; Guaman et al., 2016; Shahbaz et al.,
be one year for paraplow and vertical rotary subsoiling, 2–3 years for the 2017) and destroy newly formed aggregates. Over the long term, how­
deep plough, straight shank subsoiler, and bentleg subsoiler, and 4–5 ever, SOC storage may increase with deeper root growth and higher C
years for deep ripping. Other management practices, including cropping input (Zhang et al., 2020).
systems, soil type, and subsequent tillage, also affect the phy-subsoiling In summary, phy-subsoiling is the most commonly used subsoiling
intervals. The phy-subsoiling intervals are usually 2–3 years for wheat- type based on the mechanical breakdown of the compacted layer(s). It
maize when involving two crops a year (Tian et al., 2016), but can be has immediate effects on physical properties and confers the main
extended to 4–5 years in the case of cultivating one crop a year (Can­ benefits of deeper root growth, and efficient use of water and nutrient
arachea et al., 2000). Phy-subsoiling needs to be performed every year stocks below the common plow depth, leading to higher crop yields and
on sandy or sand loamy soils, and the intervals can be prolonged ac­ stability, especially under drought conditions.
cording to increasing clay content (Raper and Bergtold, 2007).
The main advantages of phy-subsoiling are immediate improvements 3.2. Chemical subsoiling
in soil conditions and crop yields (Table 3). Phy-subsoiling decreases soil
density, increases water penetration and storage, facilitates deeper Chemical (chem-)subsoiling is an approach for improving the
rooting, and improves the availability of water and nutrients in subsoil chemical, physical and biological properties below the common plow
to crops. depth (> 30 cm) by deep placement of various materials into the subsoil.
Phy-subsoiling presents some disadvantages, including high costs, The aims of chem-subsoiling are to make the subsoil more suitable for
accelerated SOC decomposition and aggregate destruction (Table 3). root growth and yield formation by improving the chemical properties
Every phy-subsoiling mode is machine-dependent and involves high directly and indirectly controlling the physical or biological properties
energy consumption as well as traffic-induced soil compaction. simultaneously based on additions of specific chemical materials. The

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most outstanding feature of chem-subsoiling is the placement of specific compaction by using “plant roots as a tillage tool” (Chen and Weil, 2009;
chemical materials into the subsoil which are tailored to reduce the Zhang and Peng, 2021). The second aim is to create biopores through the
unfavorable soil conditions and satisfy crop needs. The most common use of deeply penetrating tap roots (Kautz et al., 2013) and earthworms
chem-subsoiling approaches are deep incorporation of animal or green (Athmann et al., 2017; Hoang et al., 2016). These biopores provide low
manure, straw, lime, and water retention agents (Table 3). resistance pathways for the roots of succeeding crops (Banfield et al.,
Subsoil manuring means deep banding of nutrient-rich organic 2017b). The main bio-subsoilers used are deep-rooted crops or cover
amendments. Subsoil manuring at 20 t ha− 1 increased crop yields for at crops, deep straw addition, earthworms, and arbuscular mycorrhizae
least the next 4 years compared with phy-subsoiling only (Peries, 2013). fungal inoculation (Table 3, Fig. 6).
Manuring is especially useful in dense dispersive clay subsoils, which are Many deep-rooted crops, especially allorhizal root systems such as
frequently limited by high bulk density (1.5–1.6 g cm− 3), high sodicity alfalfa, carrot, and chicory, can be used as bio-subsoilers (Table 4,
(exchangeable sodium percentage > 15 %), periodic waterlogging, or Fig. 6). Deep-rooted crop subsoiling involves two stages: (1) subsoil
limited water holding capacity (Wang et al., 2020a). For example, macropore creation by roots of the “drilling” species and (2) benefits to
subsoil manuring produced large crop yield responses in sodic lands in intercropped or subsequent crops after subsoil improvements (Cresswell
southern Australia (Gill et al., 2008) and in clay-loamy soils in Canada and Kirkegaard, 1995). Roots of these crops grow deeper than 180 cm,
(Leskiw et al., 2012). Subsoil manuring decreased bulk density and and the deepest roots of alfalfa, switchgrass, and wheat even extend
increased both crop root length and density as well as macroaggregate below 300 cm (Table 4, Fig. 2). Alfalfa has the greatest root depth:
formation and stability (Peries, 2013; Wang et al., 2020a). Wheat grain 370 cm (Peixoto et al., 2020). Tap-rooted species penetrate compacted
yields increased by 3.6–5.2 t ha− 1 when alfalfa pellets were applied at layers better than fibrous-rooted species and are therefore, much better
the 30–40 cm depth using a pipe attached to a ripper compared to those adapted for use as bio-subsoilers (Chen and Weil, 2009).
applied with deep ripping only (Gill et al., 2008). Crop yields increase Taproot-multibranch crops (e.g. alfalfa) affect the pore system between
because of the prolonged greenness of leaves based on plant access to the large biopores more effectively than taproot-herringbone crops
available water and nutrients in the subsoil. Subsoil manuring also in­ (Schjønning et al., 2015). The biopores were found to be used more
creases productivity in many regions, where crops suffer severe seasonal intensively by oilseed rape roots, which were much more sensitive to
water deficits during the grain filling stage because the soil captures mechanical impedance, than by those of cereals or faba bean (Athmann
more rainfall for crops (Wang et al., 2020a). et al., 2019). More details of bio-tillage by cover crops and their effects
More than 20 % of the world’s soils suffer from subsoil acidity on soil physical properties, root growth and crop yield, were reviewed
(Rengel, 2003). Lime incorporation by a conservation subsoiler with a by Zhang and Peng (2021).
shank designed to deliver lime simultaneously at four depths down to The main advantages of deep-rooted crops in rotation or intercrop­
50 cm at 6.7 Mg lime ha− 1 reduced subsoil acidity (Blumenschein et al., ping are that they can create or enlarge existing vertical biopores and
2019). Liming of subsoils increased the pH by 0.7 units at depths of strongly increase crop root density below 30 cm (Guaman et al., 2016;
13–38 cm, with significant residual effects enduring for 2–3 years. Lime Lynch and Wojciechowski, 2015; Schjønning et al., 2015). Most of these
application in acidic soil increased the pH and calcium (Ca) content and plants originate from steppes and prairies, where droughts (and some­
consequently reduced exchangeable Al toxicity to plants (Blumenschein times nutrient limitations) are very common; these plants evolved deep
et al., 2019; Li et al., 2019). root systems for water and nutrient access. For example, wheat root
Water retention agents, known as superabsorbent polymers, can growth is almost entirely confined to biopores in dense subsoils of
absorb water at a rate 500 times their original weight (Yu et al., 2012). 1.6 g cm− 3 bulk density (depending on soil texture, however, according
Applying 105 kg ha− 1 polyacrylamide, a type of water retaining agent to Spoor et al., 2003) and 2.5 MPa penetrometer resistance (White and
(Awad et al., 2013), to a 50 cm depth in sandy soil increased the Kirkegaard, 2010). Some no-tillage farms use rotations to encourage the
moisture in the 20–80 cm layer (Wu et al., 2017). Water retention agents formation of biopores or channels (Athmann et al., 2017; Passioura,
maintain higher soil water stocks and increase dry matter production, 2002). In addition, rhizodeposits (Hafner and Kuzyakov, 2016) and dead
grain-filling rates, water use efficiencies (Guo et al., 2016b), and roots add available organic carbon, increase the abundance and activity
aggregate formation (Awad et al., 2013). Deep application of water of organisms such as earthworms and microorganisms, and increase the
retention agents resulted in more stored rainfall or irrigation water and formation of biopores and aggregate binding agents (Banfield et al.,
induced deep root growth (Zhang et al., 2016). 2017a; Hoang et al., 2017; Wang et al., 2016b).
The main disadvantage of chem-subsoiling is machine dependence Deep-rooted legumes such as alfalfa, pea, clover, and soybean fix N2
(Table 3). Chemical materials need to be applied to subsoil by phy- (Table 4), which increases the subsoil N content by N inputs from roots
subsoiling or special machines. For conservation tillage areas, custom and rhizodeposition. The soil mineral N content in the faba bean field
machines must be redesigned to distribute manure, lime, or water was approximately ten times higher than that in the winter wheat field
retention agents in subsoil with less surface soil disturbance (Blumen­ at 30–60 cm depth (Neugschwandtner et al., 2015). Clover stored more
schein et al., 2019). The second disadvantage is that chem-subsoiling N at 30–50 cm depth than maize or faba bean (Hobley et al., 2018). N
effects are slower than phy-subsoiling effects. For example, the best ef­ fixation by legumes ranges from 70 to 350 kg N ha− 1 year− 1 (Kakraliya
fects of subsoil liming are obtained one year after application (Blu­ et al., 2018), a part of which is rhizodeposited to subsoil. The N rhizo­
menschein et al., 2019). deposition of legumes was 3.4 times the N in the roots, among which
In summary, chem-subsoiling is an alternative mode to phy- 8–18 kg N ha− 1 was rhizodeposited to the subsoil (Wang et al., 2021).
subsoiling, especially in sodic, acidic, or sandy fields, aimed at con­ Subsoil N can be used by subsequent crops (Haberle et al., 2006).
trolling the chemical properties according to the needs of the crops, and Legume cropping can replace 37–77 kg N ha–1 of mineral N fertilizers
its main advantages lie in reducing acidity or sodicity and countering for wheat and maize in a rotation system (N’Dayegamiye et al., 2015).
droughts. Thus, using deep-rooted legumes to increase N storage in subsoil is very
important for the optimization of N management. The functions of
3.3. Biological subsoiling deep-rooted crops with respect to N uptake by subsequent crops, how­
ever, are much slower than those of phy-subsoiling and generally limited
Biological (bio-)subsoiling approaches use the growth, activities, by abiotic factors (Guaman et al., 2016).
rhizodeposition, and residues of crops and biota to improve subsoil Crop straw addition below 30 cm increases SOC and total N storage,
conditions for main crops. Bio-subsoiling is also known as “biodrilling” nutrient availability, and microbial activities (Table 3). Ditch-buried
(Cresswell and Kirkegaard, 1995) or “biotillage” (Zhang and Peng, straw improves soil hydrothermal properties, fosters C sequestration,
2021). The primary aim of bio-subsoiling is to offer new solutions to increases N, P, and K availability, increases microbial activity and

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T. Ning et al. Soil & Tillage Research 223 (2022) 105490

Fig. 6. Bio-subsoiling by deep-rooted crops (left and middle) and earthworms (right). A: deep-rooted crops such as carrot, alfalfa, chicory etc., can be used as bio-
subsoilers. Tap-rooted species penetrate compacted layers better than fibrous-rooted species. Taproot-multibranch crops enlarge the pore system with more and
larger biopores than taproot-herringbone roots. B: Subsequent crops can use the remaining biopores (white parts in soil) created by preceding deep-rooted crop. C:
Anecic earthworms are vertical burrowers, which can create vertical biopores up to 2.4 m deep. Old and new burrows can be used by growing crops to uptake water
and nutrients below plough pan. The loupes (B and C) show the new roots growing in the depth through the old biopores (B: root biopore; C: earthworm biopore).

functional diversity, hinders the occurrence of common pests and accelerates the formation of aggregates and increases nutrient avail­
pathogens, and ultimately increases crop yields (Yang et al., 2019). ability and organic C contents (Agnihotri et al., 2022; Lino et al., 2018).
Deeper burying of straw in a rice-wheat rotation system helps control Subsoil AM fungi increase fertilizer efficiency and C sequestration and
the occurrence of wheat Fusarium culmorum and rice Sclerotium oryzae, reduce greenhouse gas emissions (Sosa-Hernández et al., 2019). Thus,
as well as Fusarium head blight of wheat and Chilo suppressalis infestation increasing subsoil AM communities by crop rotation with deep-rooted
in rice (Tian et al., 2019; Yang et al., 2019). In practice, the key limi­ mycorrhizal crops and/or by inoculation with AM fungi into deep soil
tation for the application of ditch-buried straw is the lack of represents a new method of bio-subsoiling.
high-efficiency machinery (Yang et al., 2019). In summary, bio-subsoiling is an eco-friendly approach that slowly
Soil animals, such as anecic earthworms (Fig. 6), can also be used as increases the biologically formed pores (biopores) and biodiversity in
bio-subsoilers because they increase the formation of biopores and ag­ the subsoil.
gregates (Athmann et al., 2017; Hoang et al., 2017; Lino et al., 2018;
Sosa-Hernández et al., 2019). Earthworm abundance ranges between 5
and 150 individuals per m2 in the ecosystems of 57 countries (Phillips 3.4. Combination of subsoiling approaches
et al., 2019). Earthworms increase biopore numbers, enzyme activities,
and microbial activities by burrowing and casting (Bertrand et al., 2015; To increase their advantages and reduce their disadvantages, phy-,
Hoang et al., 2017; Karaca, 2011). Burrow size and continuity depend on chem- and bio-subsoiling methods should be used in combination. Deep
the ecological strategies of earthworms: namely, epigeic, endogeic, or tillage combined with deep-rooted subsequent crops, such as alfalfa, red
anecic species (Edwards, 2004; Edwards and Bohlen, 1996; Karaca, clover and radish, is one type of phy- and bio-subsoiling combination
2011). Anecic species, such as Lumbricus terrestris, can create single and (Guaman et al., 2016). Combined phy- and bio-subsoiling decreased soil
nearly vertical channels up to 12 mm in diameter and 2.4 m deep resistance by more than 2.5 MPa down to 40 cm and by 1.0 MPa down
(Blouin et al., 2013; Edwards and Bohlen, 1996), and the annual burrow to 50 cm as compared with the control (Fig. 7). Combining
length is estimated to be 80 km ha− 1 (Karaca, 2011). Anecic earthworms phy-subsoiling with bio-subsoiling slightly decreased soil resistance at
can also carry certain materials, such as lime or straw, down to the the 50–60 cm depth compared to phy-subsoiling (Fig. 7). Crop root
subsoil by either direct bodily attachment or via ingestion and subse­ density after combined subsoiling was higher than that in the context of
quent deposition (Chan, 2011). Thus, fostering earthworm populations separate inter-row or biological subsoiling (Guaman et al., 2016).
in subsoil by using earthworm inoculation and no-till or organic farming Growing alfalfa as a bio-subsoiling agent or incorporating slaked lime
systems (Karaca, 2011) could be a beneficial method for bio-subsoiling. for chem-subsoiling with phy-subsoiling prolonged the positive effects
Higher earthworm abundance and biomass in fields requires the appli­ by up to 2 years (Löfkvist, 2005). Chem-subsoiling also needs to be
cation of organic fertilizers such as crop residues, farmyard manure, and combined with phy-subsoiling or other tillage methods to allocate the
green manure (Elyamine et al., 2018; Sizmur et al., 2017). Straw milled chemicals into the subsoil (Blumenschein et al., 2019). Combined
to < 3 mm increases earthworm populations and accelerates their phy-subsoiling with bio-subsoiling or chem-subsoiling manifests clear
growth (Sizmur et al., 2017). The inoculation of earthworms in arable positive and prolonged effects upon soil quality and crop yields (Gua­
soil increased microbial biomass and enzyme activities in biopores and man et al., 2016). More combined modes should be studied to determine
boosted nutrient mobilization into the subsoil (Athmann et al., 2017). the best use of subsoiling in sustainable agronomy.
However, the earthworm inoculation technique is not widely used to
date because of a number of factors, including the selection of earth­ 4. Applications of subsoiling
worm species as determined by site conditions and objectives, timing
determined by soil maturity and season, and aftercare/monitoring 4.1. Effects of subsoiling on crop yields and SOC storage
(Karaca, 2011). This makes it important to foster earthworm pop­
ulations with other bio-subsoiling modes, such as deep-rooted crops and A meta-analysis of papers published over the past 10 years was used
straw retention, to increase their efficiency. to compare subsoiling vs. no-till effects on crop yield, bulk density, SOC
Increasing soil fungi, such as arbuscular mycorrhizae (AM), storage, and water use efficiency. All data were collected from the Web
of Science based on the keywords “(crop name) AND (subsoiling OR

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Table 4 deep till* OR deep plo*)”. The average ratio of crop yield under sub­
Agronomic properties of the main deep-rooted crops used in agricultural systems soiling to that under no-till was always larger than 1.0, with the highest
for biological subsoiling. value for wheat (1.26) and the lowest for potato (1.07) (Fig. 8 left). The
Common and Root system, Advantages Disadvantages average yield increase under subsoiling was 19 % higher than that under
(botanical) effective/ no-till (Fig. 8 right). Although potato has the lowest yield surplus,
name maximum subsoiling increased tuber size, and thus potato price (Copas et al.,
root depth
2009). The lower potato yield response to subsoiling primarily occurred
Alfalfa Tap root Perennial, N2 > 2 years are in sandy soils and irrigated fields (Copas et al., 2009). The highest yield
(Medicago 60/370 cm fixation, very deep necessary, nematodes
ratio of subsoiling to that under no-till for wheat reached up to 1.65 in
sativa) roots, strongly are increased, high
developed root water demand, high Udolls in a semihumid climate (Tian et al., 2016) and 1.63 in clay loamy
system, many nutrient demand (K, soil in a humid subtropical climate (Alam et al., 2014). The ratio for
varieties with P, Ca), not wheat was higher in silty loam than in silty clay loam (Jug et al., 2019).
specific adaptations, appropriate for acidic Thus, subsoiling confers the largest advantages in loamy soils in semi­
development of soil soils
fauna
humid and humid climates.
Chicory Tap root Perennial, fodder > 2 years are The SOC contents after subsoiling are similar to or lower than those
(Cichorium 90/200 cm crop, 2/3 roots in necessary, greater with no-till (Fig. 8 right). This is mainly because phy-subsoiling accel­
intybus) subsoil, the number water uptake from erated organic matter decomposition, although subsoil was mixed with
of biopores per area soil
topsoil with a higher SOC content (Canarachea et al., 2000; Guaman
unit in 45–65 cm soil
depth exceeds that in et al., 2016), thus reducing SOC storage over the short and medium
topsoil terms (Zhang et al., 2020). Bio-subsoiling, however, increased SOC
Pea Tap root N2 fixation, many Cannot be followed storage due to high C inputs from straw return (Singh et al., 2019; Tian
(Pisum 20/160 cm varieties with by legumes, high et al., 2016) and from deep roots (Osanai et al., 2020; Poffenbarger
sativum) specific adaptations, nutrient demand (K,
et al., 2020). Consequently, this increased C input in the subsoil will
cash crop, can grow P, Ca)
in barren soil likely increase SOC storage over the long term, especially considering
Red clover Fibrous root Perennial, N2 High water demand, the increased efficiency of microbial C stabilization with depth (Peixoto
(Trifolium 30/240 cm fixation, high-quality high nutrient demand et al., 2020). Thus, suitable subsoiling can achieve long-term effects on
pratense) feed crop, large (K, P, Ca), not heat
higher yield and SOC storage when bio-subsoiling is part of
distribution, adapted resistant
to pH of 6–7.5 management.
Rye Fibrous root, Gramineous crop, Not heat resistant The average water use efficiency is 11 % higher under subsoiling
(Secale 60/230 cm deep roots, reduces (Fig. 8 right) because of increased total porosity and water storage
cereale) soil compaction (Lamptey et al., 2017), as well as occupation of deeper soil volume by
Sugar beet Tap root, Biennial, important High water demand,
roots. Subsoiling increased the crop yield in 60 % of the observations
(Beta 60/180 cm vegetable, cash crop, high nutrient demand
vulgaris) temperature-tolerant (N, P, K), not among 1530 paired comparisons to conventional tillage (Schneider
but more resistant to appropriate for acidic et al., 2017). Total tuber yield was 36 % higher with subsoiling at depth
cold soils up to 70 cm than under conventional tillage (Costa et al., 2017). Because
Sunflower Tap root, Uptake of toxic High water demand,
the root percentage increased in subsoil compared to that under con­
(Helianthus 40/200 cm ingredients in soil, high nutrient demand
annuus) very tall shoots, (N, P, K), serious
ventional tillage (Cai et al., 2014; Sun et al., 2017), the crop yield and
beautiful flowers weed or disease SOC stocks also increased (Feng et al., 2018; Tian et al., 2014).
Switchgrass Fibrous root, Perennial, high > 2 years are
(Panicum 30/300 cm biomass, bioenergy necessary, serious 4.2. Subsoiling practices
virgatum) crop, less need for weed
fertilizer and
irrigation, can grow For the best agronomic approaches, farmers should consider whether
in barren and high- subsoiling is needed, which mode should be chosen, and how to apply it
salinity soil optimally. The application of subsoiling should involve consideration of
three main factors: soil properties, crops, and climate (Fig. 9). First, the
subsoiling modes should be determined based on the soil type,
compaction level and depth. Subsoiling will significantly increase crop
yields from loamy or clay soils with high compaction (> 1.5 g cm− 3) as
well as acidic and alkaline soils (Fig. 9). Subsoiling is better when more
nutrients are stored in the subsoil, and less in the topsoil. Second, a
crop’s root architecture and its water and nutrient needs should be
considered. For example, phy-subsoiling is especially useful for wheat,
barley, and maize (Fig. 9).
Third, the climate should be considered. Subsoiling is more suitable
in semiarid and semihumid than in humid regions (Akinci et al., 2004;
Sojka et al., 1997; Tian et al., 2016) and more suitable in rainfed than
irrigated fields (Fig. 9). The reasons for this are: 1) in humid regions,
abundant soil water reduces the negative effects of soil compaction, and
crop roots have less need to grow deeper to access water or nutrients; 2)
in semihumid and semiarid regions, soil compaction will be aggravated
during alternating wet and dry periods, and storing more water by
subsoiling is important for farmers; and 3) in arid regions, especially
Fig. 7. Soil resistance during vertical penetration after physical and / or bio­ during the dry season, topsoil water storage is so limited that subsoil
logical subsoiling. Physical subsoiling is 10–20 times more efficient than bio­ storage is urgently necessitated.
logical subsoiling to decrease penetration resistance. The subsoiling modes should be decided according to the main
(modified from Guaman et al., 2016). limiting factors for the crops (Fig. 10). If physical factors such as density

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T. Ning et al. Soil & Tillage Research 223 (2022) 105490

Fig. 8. Ratios of physical subsoiling vs. no-till for yields of the 6 most important crops (left) and soil organic carbon (SOC) contents in the top 60 cm depth (0–60 cm,
right). The red and blue numbers on the right plot are the average values and the number of matched data pairs, respectively. Red dashed line (X = 1.0) corresponds
to the absence of effects by subsoiling as compared to no-till. WUE: water use efficiency. All data were collected from the Web of Science (from 2011 to 2020) based
on the keywords “(crop name) AND (subsoiling OR deep till* OR deep plo*)”.

the crop, cropping system, and clay content. For perennial crops and
soils with a high clay content, the subsoiling interval is 4–6 years. The
shank distance depends on the degree of soil compaction. For wheat and
maize, the best lateral distance between the shanks is 60 cm (Raper and
Bergtold, 2007; Tian et al., 2016).

4.3. Subsoiling diversification

Combinations of phy-subsoiling, bio-subsoiling, and/or chem-

subsoiling are encouraged to optimize crop yield and SOC storage.
Phy-subsoiling alone is a necessary but insufficient basis for sustainable
solutions to the compaction problem, which ought to be prevented
rather than repaired (Schjønning et al., 2015). Thus, the general prin­
ciple should be less use of phy-subsoiling alone, but more use of bio- and
Fig. 9. Subsoiling efficiency related to properties of soils, crops, and climate. chem-subsoiling combined with phy-subsoiling or conservation tillage.
The subsoiling modes should be determined based on soil type, compaction When tillage is inevitably needed, phy-subsoiling should be imple­
level and depth. Subsoiling will strongly increase crop yield on loamy or clay mented with lighter and less powerful machines, and the operation time
soils with high compaction, as well as for acid and alkaline soils. Physical should be reduced. In ecofarms, bio-subsoiling by deep-rooted crop ro­
subsoiling is especially useful for wheat, barley, and maize. Subsoiling is more tations along with improving the conditions for earthworms and AM
suitable in semi-arid and semi-humid than in humid regions, and more suitable fungi to thrive will effectively reduce compaction and help sustain soil
in rainfed than irrigated fields. health. Thus, we propose a new concept of “subsoiling diversification”,
which means including bio- and chem-subsoiling along with phy-subsoiling.
(Bt horizon or tillage pan) or porosity are the main limiting factors, phy- Subsoiling diversification can make best use of the advantages and
subsoiling should be chosen. If chemical factors such as pH and nutrients bypass the shortcomings of individual subsoiling practices. This
are the main limiting factors, e.g., in acidic or alkaline soils, Gleysols, approach promises wide and valuable future application prospects.
Arenosols, and Cambisols, then chem-subsoiling should be chosen. If soil
biota such as earthworms or microorganisms are the main limiting 5. Conclusions & Outlook
factors, bio-subsoiling should be chosen in order to supply more food to
biota by providing more organic materials or planting deep-rooted We reviewed the approaches along with the advantages and short­
crops. In most cases, bio- or chem-subsoiling should be combined with comings of subsoiling applications and suggested several optimization
phy-subsoiling to decrease soil compaction and maintain yield stability strategies. Subsoil plays an important but often overlooked role in sus­
over longer periods. tainable crop production and eco-farming systems (Kautz et al., 2013).
After the mode is determined, four options of phy-subsoiling should Subsoiling is necessary to help crop roots grow through compacted
be decided: subsoiler type, subsoiling time and depth, shank distance, layers to access more than half of the nutrients and water stored below
and periodicity (Fig. 10). First, the choice of a suitable subsoiler is 30 cm.
crucial for optimal crop benefit and cost savings (Fig. 5). Selecting Applying bio- and chem-subsoiling combined with phy-subsoiling is
straight shanks decreases energy requirements because it only mini­ an urgent matter and crucial to reduce costs and increase environmental
mally disturbs the soil surface as compared to other subsoilers (Raper, and economic benefits. Phy-subsoiling – the most widely studied and
2007). Second, the subsoiling time, depth and tine distance must be used method – yields immediate and strong effects for hardpan loos­
decided. A depth of 35–45 cm is optimal in most (but not all) cases ening. In contrast, bio-subsoiling and chem-subsoiling have been dis­
because the compacted layer – the tillage pan – is commonly located at regarded because their effects are weaker and slower (especially those of
this depth (Raper, 2007). Deeper subsoiling is necessary for naturally bio-subsoiling), and in most cases, combination with phy-subsoiling
dense Bt horizons, especially of Planosols, Stagnosols, Acrisols, Lixisols, approaches is needed. Chem-subsoiling can help control the soil pH
Luvisols, Solonetzes, and Vertisols. The subsoiling periodicity is and increase nutrient availability. Bio-subsoiling is an eco-friendly mode
commonly 2–4 years when needed repeatedly, with decisions based on for increasing SOC and the biodiversity of soil animals and

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T. Ning et al. Soil & Tillage Research 223 (2022) 105490

Fig. 10. Practice guides and future research work for better use of subsoiling. The main limiting factors should be used to determine the single or respective
combination of subsoiling approaches. The ellipses show that other factors in the corresponding group are possible. The boxes for future work show which are the
most important for farmers (green), industry (blue), and researchers (yellow).

microorganisms. We suggest using bio-subsoiling modes, including ro­ enable farmers to use tillage to greater benefit crop yields and
tations with deep-rooted crops or intercropping, deep straw addition, SOC storage.
and chem-subsoiling modes, including subsoil manuring and lime (3) Mechanisms of subsoiling functions. Increased efforts should be
incorporation. Highly efficient subsoiling machines and equipment are made to study the effects of phy-, chem-, and bio-subsoiling on
necessary, especially those enabling the combined application of soil properties, especially on aggregate and pore formation, long-
chemical and biological approaches. term stability, organic matter accumulation, hotspots of micro­
Three fundamental needs require further investigations for better bial activities, and nutrient and water use. The mechanisms un­
theoretical understanding and improved practical application to maxi­ derlying these effects may differ from those in topsoil and require
mize the advantages and minimize the disadvantages of these methods more scrutiny.
(Fig. 9):
Declaration of Competing Interest
(1) New modes of chem- and bio-subsoiling. For chem-subsoiling,
more chemical materials should be studied to control subsoil The authors declared that they have no conflicts of interest to this
acidity, salinity, alkalinity, water storage, anoxic conditions, and work.
heavy metal toxicity in certain regions. For bio-subsoiling, more
attention should be given to soil animals and microbial functions Acknowledgements
in relation to subsoil properties and the most efficient ways to use
them. This also calls for innovative biochem-subsoiling modes to This work was supported by the Shandong Major Science and
achieve the best agronomic practices. Technology Innovation Projects, China,(2019YQ014,
(2) Subsoiling diversification. For the best agronomic practices, we 2020CXGC010803, and 2021CXGC010804), and the “RUDN University
suggest new approaches to subsoiling combinations. Further­ Strategic Academic Leadership Program, Russia”.
more, global compaction maps (Nachtergaele et al., 2010), sub­
soiling maps, and tillage diversity maps should be drawn to

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T. Ning et al. Soil & Tillage Research 223 (2022) 105490

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